Many vaccine compositions include immunological adjuvants in order to increase antigenic potency. Immunological adjuvants act to augment cell-mediated and humoral immune responses. Such adjuvants include depot adjuvants, compounds which adsorb and/or precipitate administered antigens and which serve to retain the antigen at the injection site. Typical depot adjuvants include aluminum compounds and water-in-oil emulsions. However, depot adjuvants, although increasing antigenicity, often provoke severe persistent local reactions, such as granulomas, abscesses and scarring, when injected subcutaneously or intramuscularly. Other adjuvants, such as lipopolysacharrides and muramyl dipeptides, can elicit pyrogenic responses upon injection and/or Reiter's symptoms (influenza-like symptoms, generalized joint discomfort and sometimes anterior uveitis, arthritis and urethritis). Accordingly, there is a continued need for effective and safe adjuvants for use in a variety of pharmaceutical compositions.
Saponins are high molecular weight, glycosidic, natural plant surfactants, consisting of an aglycone ring linked to one or more sugar chains. Although saponins share common properties, they are structurally diverse. For example, the aglycone can be steroid, triterpenoid or a steroidalalkaloid and the number of sugars attached to the glycosidic bonds varies greatly.
Numerous pharmacological properties have been attributed to saponins, including anti-inflammatory (Shibata, S. (1977) in New Natural Products and Plant Drugs with Pharmacological, Biological or Therapeutic Activity (Wagner, H. and Wolff, P. eds, Springer, Berlin) pp. 177-196); antiviral (Amoros et al., Antiviral Res. (1987) 8:13-25); molluscicidal (Hostettmann, K., Helv. Chim. Acta. (1980) 63:606-609); contraceptive (Bhargava, S. K., Int. J. Crude Drug Res. (1988) 26:229-233); antibacterial (Chowdhury et al., J. Bangladesh Acad. Sci. (1987) 11:75-82); and fungicidal activities (Anisimov et al., Izv. Akad. nauk SSSR, Ser. Biol. (1979), 570-575 (Chem. Abstr. 107:108725).
More recently, saponins have been found to exhibit adjuvant and immunostimulating properties. See, e.g., Kensil et al., J. Immunol. (1991) 146:431-437. For example, the triterpene glycoside saponins extracted from the South American tree, Quillaja saponaria, termed Quil A (U.S. Pat. No. 5,057,540; International Publication No. WO 88/09336, published Dec. 1, 1988) have been used as immunological adjuvants in vaccine compositions against a variety of infectious diseases.
Roots of the plant Polygala senega (commonly known as "snakeroot"), contain at least 6-10 triterpenoid saponins. Hostettmann and Marston, 1995 in Chemistry and Pharmacology of Natural Products: Saponins (Phillipson, J. D., ed. Cambridge University Press, New York) p. 323, the main saponins being senegin II, III and IV (Shoji et al., Chem. Pharm. Bull. (1973) 21:791-799; Tsukitani and Shoji, Chem. Pharm. Bull. (1973) 21:1564-1574; and Yoshikawa et al., Chem. Pharm. Bull. (1995) 43:350-352. These saponins differ from Quil A saponins in (a) the number of sugars present in the side chains of the molecules, and (b) the presence of different functional groups in the triterpenoid aglycone. Thus, the P. senega saponins are chemically distinct from the Quillaja saponaria saponins.
P. senega saponins have been reported to exhibit hypoglycemic effects (Kako, et al., Biol. Pharm. Bull. (1995) 18:1159-1161 and Yoshikawa et al., Chem. Pharm. Bull. (1995) 43:2115-2122); reduce blood triglyceride levels (Masuda et al., Biol. Pharm. Bull. (1996) 19:315-317); and reduce alcohol absorption (Yoshikawa et al., Chem. Pharm. Bull. (1995) 43:350-352 and Yoshikawa et al., Chem. Pharm. Bull. (1995) 43:2115-2122). However, the use of Polygala senega saponins as immunological adjuvants has not been previously described.